1. Field of the Invention
This invention relates to an information transmission method capable of reproducing an information signal recorded on a recorded medium, transmitting the same together with duplication prohibiting information, receiving the transmitted information signal and limiting or prohibiting the information signal from being recorded on another recording medium, as well as an information duplication prohibiting method, an information duplication prohibiting device and an information recording medium.
2. Related Art Statement
VTRs (Video Tape Recorders) have been popularized and many softwares which can be played back on VTRs have been provided. Further, playback devices such as digital VTR or DVD (Digital Video Disc) have been put to practical use recently and high quality video images or sounds can be easily enjoyed by playing back them.
There is, however, a problem that such softwares provided in abundance are duplicated without authorization and several methods have been utilized for prohibiting such duplication.
For example, in a case of using VTRs for outputting analog video signals, there is an anti-duplication method of utilizing a difference of the AGC (Automatic Gain Control) system or a difference of the characteristics of the APC (Automatic Phase Control) between a VTR as the recording device and a monitor receiver for providing video images.
In a case of utilizing the difference of the AGC system, for example, when the VTR adopts AGC that use a pseudo sync signal inserted in the video signal while the monitor receiver adopts an AGC systems that does not use such a pseudo sync signal, a pseudo sync signal at an extremely high level is inserted into the video signal supplied from the play back VTR to the recording VTR upon outputting the same.
Further, in a case of utilizing the difference of APC characteristic, when the VTR adopts APC that use to the phase of the color burst signal in the video signal, while the monitor receiver adopts an APC system different therefrom, the phase of the color burst signal in the video signal supplied from the playback VTR to the recording VTR is partially inverted.
Thus, the monitor receiver receiving the analog video signal supplied from the playback VTR can normally playback the video image free from the effect of the pseudo sync signal or the partial inversion of the phase for the color burst signal used for the APC.
Then, the VTR that receives the analog video signal inserted with the pseudo sync signal or undergoing the phase inversion control of the color burst signal as described above and records the signal on the recording medium can not conduct normal gain control or phase control based on the input signal and can not normally record the video signal. Accordingly, even if the recorded video signal is played back, normal video image capable of being viewed can not be played back.
Further, a digital VTR or the like used for digitized video signals conducts anti-duplication control such as prohibition of duplication by adding an anti-duplication control signal comprising, for example, an anti-duplication control code or a duplication ranking controlling code to the video signal as data and recording the same on the recording medium.
In this case, the digital VTR as the play back device reads out the video signal, audio signal and anti-duplication control signal recorded on the recording medium and supplies them as a digital or analog play back signal to the digital VTR as the recording device.
The digital VTR as the recording device extracts the anti-duplication control signal from the supplied playback signal, and controls recording the supplied play back signal to the recording medium based on the anti-duplication control signal. For example, if the anti-duplication control signal contains a duplication prohibition code, the digital VTR as the recording device conducts control so as not to conduct recording.
Further, if the anti-duplication control signal contains a duplication ranking control code, recording is controlled in accordance with the ranking control code. For example, if the ranking control code is information for permitting copying only once, the digital VTR as the recording device adds a duplication prohibition code and records the video signal and the audio signal as the digital data in recording medium. Accordingly, the video signal can not be duplicated by using such a duplicated recording medium.
In the so-called digital connection of supplying the video signal, audio signal and anti-duplication control signal as digital signals to the digital VTR as the recording device, when the anti-duplication control signal in the form of digital data is supplied to the digital VTR as the recording device, anti-duplication control can be conducted using the anti-duplication control signal on the side of the recording device.
However, in the analog connection of supplying the video signal and audio signal as analog signals to the digital VTR as the recording device (A/D conversion being conducted by digital VTR), since the anti-duplication control signal is not superimposed on the analog information signal such as the video signal or audio signal, when the signals supplied to the recording device is put to D/A conversion, the anti-duplication control signal is depleted.
Therefore, in the case of the analog connection, an anti-duplication control signal has to be added to the D/A converted video signal or audio signal, which deteriorates the video signal or audio signal.
That is, it is difficult to add the anti-duplication control signal and take out the same in the recording device for use in anti-duplication control, without deteriorating the D/A converted video signal or audio signal.
In view of the above, anti-duplication has been conducted so far in the case of the analog connection by adopting the anti-duplication control method utilizing the difference of the AGC system or the difference of the APC characteristics between the VTR and the monitor receiver as described previously.
However, the duplication prohibition method by utilizing the difference of the AGC system or the difference of the APC characteristics between the VPR and the monitor receiver involve the problem that the video signal is recorded normally and anti-duplication can not be attained or the played back image on the monitor receiver is disturbed depending on the AGC system or the APC characteristics on the side of the-recording device. Further, it is troublesome to adopt anti-duplication methods between the analog connection and the digital connection as described above.
As of an anti-duplication method which is effective in both the analog connection and the digital connection and does not deteriorate the played back video image or sound, a method of spectrally spreading the anti-duplication control signal and superimposing the same on the video signal has been considered.
In this system, a PN (Pseudo Random Noise) series code (hereinafter referred to as PN code) used as the spreading code is generated at a sufficiently rapid period, which is multiplied by anti-duplication control signal, thereby converting the duplicate control signal which is spectrally spread and has narrow band and high level into a wide band low level signal giving no undesired effect on the video signal or the audio signal. Then, the spectrally spread anti-duplication control signal is superimposed on the analog video signal and then recorded in the recording medium. In this case, the signal to be recorded on the recording medium may be either a analog signal or a digital signal.
Not in a case of a recording medium in which the spectrally spread anti-duplication control signal is superimposed on the video signal but in a case of a recording medium in which the anti-duplication control signal is recorded together with the video signal by another method, the anti-duplicate control signal is extracted from the played back signal in the play back device, applied with a PN code to spectrally spread, and superimposed on the outputted video signal.
On the other hand, the recording device conducts inverse spectral spread to the video image inputted therein by applying phase control so as to generate a PN code which is identical in the generation timing and the phase with those the PN code used for the spectral spread of the anti-duplication control signal, and multiplying the PN code with the video signal superimposed with the spectrally spread anti-duplication control signal, thereby taking out the original anti-duplication control signal. Then, the duplication control is conducted based on the anti-duplication control signal taken out by the inverse spectral spread.
As described above, since the anti-duplication control signal is spectrally spread and superimposed as a wide band and low level signal on the video signal, it is difficult for those who are unauthorized to conduct duplication to eliminate the superimposed anti-duplication control signal from the video signal.
However, the anti-duplication control signal superimposed by the inverse spectral spread can be detected and utilized. Accordingly, the anti-duplication control signal can be provided together with the video signal reliably to the recording device, and the recording device can detect the anti-duplication control signal and conduct duplication control reliably in accordance with the detected anti-duplication control signal.
By the way, it is theoretically possible, although difficult, to eliminate the anti-duplication control signal spectrally spread and superimposed on the video signal from the video signal as described above.
For instance, if the spectrally spread anti-duplication control signal is at a constant level, after the video signal is received and before the video signal is supplied to the recording device, the spectrally spread anti-duplication control signal superimposed on the video signal is detected, for example, by using a sliding correlator. Then, an offset signal at the same level and the same phase as those of the spectrally spread anti-duplication control signal superimposed on the video signal is generated based on the detected anti-duplication control signal, and the generated signal is subtracted from the video signal superimposed with the spectrally spread anti-duplication control signal, thereby enabling the anti-duplication control signal which is spectrally spread and superimposed on the video signal to be eliminated.
Then, if the anti-duplication control signal superimposed on the video signal is eliminated, duplication of the video image which is prohibited is possible and it may be that unauthorized duplication of the video image can be conducted.
In view of the foregoings, it is an object of this invention to make an anti-duplication control signal more difficult to be eliminated in a case of transmitting a spectrally spread anti-duplication control signal that superimposed on the information signal, without deteriorating the information signal.
The foregoing object can be attained in accordance with this invention by the information transmission method comprising;
modulating an anti-duplication control signal for prohibiting or permitting duplication of information signal on a recording medium, into a spectrally spread signal that has a changing at the level and
superimposing the modulated anti-duplication control signal on the information signal and transmitting the same.
Further, in the preferred embodiment for the transmission method described above wherein
the information signal is a video signal and
a spreading code used for spectral spread of the anti-duplication control signal is generated in synchronization with the video synchronization signal in the video signal.
According to the information transmission method of this invention as described above, the anti-duplication control signal is modulated into a spectrally spread signal that has a changing level, and transmitted while being superimposed on the information signal.
In this case, the level of the spectrally spread anti-duplication control signal is changed at random, for example, by using an optional random number. Accordingly, unless the random number used for the level change is known, it can not be recognized how the level of the anti-duplication control signal is changed.
Accordingly, even if it is intended to eliminate from the information signal the anti-duplication control signal superimposed on the information signal, it is not possible to form a signal for offsetting the spectrally spread anti-duplication control signal that is changing at the level and it is further difficult to eliminate the anti-duplication control signal superimposed on the information signal.
On the other hand, the inverse spectral spread treatment can be conducted with no significant effect by the level change of the anti-duplication control signal and without using the random number used for changing the level.
Accordingly, the anti-duplication control signal superimposed on the information signal is not eliminated and the anti-duplication control signal superimposed on the information signal can be extract for conducting anti-duplication control in accordance with the anti-duplication control signal.
Further, in the preferred embodiment for the information transmission method of this invention described above, if the information signal is a video signal, the spread code used for spectral spread is generated, for example, in synchronization with a vertical sync signal which is a video sync signal in the video signal.
Thus, also in the receiving side of the video signal as the information signal, a spreading code for inverse spread can be generated at the same timing as that of the spreading code used for spread.
Accordingly, the code used for spread spectrally spreading the anti-duplication control signal superimposed on the video signal as the information signal can be rapidly synchronized with the spreading code for inverse spread, and the anti-duplication control signal superimposed on the video signal can be detected rapidly and exactly.